1. Field of the Invention
The present invention relates to a rotor for a steam turbine and a method of manufacturing the same.
2. Description of the Related Art
Since 9-13% Cr content group heat resisting steels (for example, there are steels of 11% Cr—1% Mo—0.6% Ni—0.7% Mn—0.2% V—0.3% Si—0.2% C—0.1% Nb—0.06% N—the balance being Fe all by weight, and steels of 11% Cr—2.6% W—0.2% Mo—2.5% Co—0.5% Ni—0.5% Mn—0.2% V—0.05% Si—0.1% C—0.1% Nb—0.03% N—0.02% B—the balance being Fe) have high temperature strength and low temperature toughness, they have drawn attention as a material for high and intermediate pressure rotors of a steam turbine, and their use is expanding. Because the turbine rotors that rotate at a high speed are supported by a sliding bearing, sliding characteristics of the rotor material influence endurance of the bearing part.
Although 9-13% Cr heat resisting steels have excellent mechanical properties as a rotor material, the sliding characteristics are poor. It is reported that a destructive accident at a position between a journal part and a bearing metal tends to occur (Non-patent document No. 1).
Particularly, so-called “wire wool damage” tends to occur wherein the surface of the journal is scraped leaving fine stripes as if the surface were machine-worked, and there is damage wherein coil-form fine lines in generated foreign matter are found.
A cause of the damage in the journal is thought to be the inclusion of foreign matter between the journal and the bearing metal. Especially, since the 9-13% Cr heat resisting steel has small thermal conductivity, local sticking may occur when the foreign matter enters. Further, since the amount of Cr is large, Cr carbides may be produced when the temperature elevates at the time the foreign matter enters so that the carbides become another foreign matter, which promotes further damage of the journal.
In order to prevent the damage of the journal of the steam turbine rotor made of 9-13% Cr heat resisting steel, there was proposed a method wherein a deposit welding layer of low alloy steel with a small amount of Cr is coated on the journal alloy (Patent document No. 1).
Further, there was proposed a method wherein the deposit welding layer is composed of upper and lower layers, in which the lower welding layer has a lower tensile strength and a larger coefficient of thermal expansion than those of the upper welding layer so that a residual stress remaining in the welding layers is made small (Patent document No. 2).    Patent document No. 1: Japanese patent laid-open 57-137456    Patent document No. 2: Japanese patent laid-open 06-272503 Non-Patent document No. 1: “Damage in Journal”, Thermal Power Plant, Vol. 23, No. 5, pp. 536-542, published May 1972
However, in the case where the low alloy steel contains a smaller amount of Cr and has better sliding characteristics than the 9-13% Cr heat resisting steel, since the thermal expansion coefficient of the 9-13% Cr heat resisting steel is smaller than that of the low alloy steel, there remains a tensile residual stress in the surface of the deposit welding layer.
Accordingly, there were problems that cracks tend to occur in the deposit welding layer or welding heat-affected zones, etc. at the time of welding, post heat treatment, usage or in service.
In the methods which employ the deposit welding, a Cr content of the deposit welding layer increases due to dissolution of Cr (dilution) at welding from the base material, i.e., 9-13% Cr heat resting steel.
Therefore, it is necessary to make a thickness of the welding layer such that the surface of the welding layer is not affected by the dilution, which, on the other hand, may cause welding cracks.
In addition, the thick deposit welding and post heat treatment make the process expensive and not productive.